1. Field of the Invention
The present invention generally relates to a continuously variable transmission. More particularly, the present invention relates to a small vehicle, such as a motorcycle or small all-terrain four-wheel vehicle, for example, having a continuously variable transmission.
2. Description of the Related Art
In small scooter-style motor vehicles, a rubber V-belt continuously variable transmission is used to transmit power from an engine to the drive wheel. The rubber V-belt is entrained around a driving pulley, which is mounted to the crankshaft, and a driven pulley, which is in mechanical communication with the drive wheel, which is usually the rear wheel. The power of the engine is transmitted to the rear wheel by the V-belt. Each of the driving pulley and driven pulley has of a fixed sheave located on one side in an axial direction of the shaft on which it is mounted and a movable sheave on the other side.
The movable sheave of the driving pulley moves in the axial direction when a centrifugal roller is moved in the radial direction, and the movable sheave of the driven pulley is urged toward the fixed sheave by a compression coil spring. Thus, centrifugal movement of the roller, which is a function of shaft speed, closes the driving pulley while the spring force closes the driven pulley. When the driving pulley is closed, the driven pulley opens and when the driving pulley slows in rotational speed, the driven pulley closes, which opens the driving pulley. In a continuously variable transmission of this type, when the movable sheaves move in the axial direction, the diameter at which the V-belt engages with the pulleys varies and the transmission ratio is continuously varied. In the closed position, the diameter is larger than the diameter in the open position.
Unfortunately, transmissions using rubber V-belts have operational characteristics that limit the horsepower of the engines with which they have utility. In other words, the rubber V-belts have a limited transmission capacity.
The transmission capacity can be increased through the use of a steel belt instead of the rubber V-belt. Steel belts, however, require strengthening of most, if not all, of the rotating parts because of the tension required for the belts and because of the increased weight of the belts. Thus, while steel belts can increase the transmission capacity, the steel belts disadvantageously raise the weight and size of the transmission.
Other forms of continuously variable transmission include torque converter-type transmissions used in passenger cars, toroidal-type transmissions in which an axial angle variable roller is interposed between power transmitting faces opposed to each other, and ball-type continuously variable transmissions, which are discussed below.
Because the torque converter-type and toroidal-type continuously variable transmissions are larger in size and weight than belt-type continuously variable transmissions, they seldom are mounted on small vehicles, such as motorcycles, that require ease in handling.
Japanese Patent Publication No. JP-A2002-513889 (FIG. 1) discloses a ball-type continuously variable transmission. The continuously variable transmission disclosed in the reference has an inner ring mounted on an input shaft, an outer ring located radially outside the inner ring, and a plurality of transmitting balls held between the inner and outer rings. The balls roll between the inner and outer ring.
Each of the inner and outer rings has a first half and a second half that are aligned in the axial direction. In other words, the inner ring has a first half that is generally axially aligned with a first half of the outer ring and the out inner ring has a second half that is generally axially aligned with a second half of the outer ring. Each of the first and second halves of the inner and outer rings defines a concave surface extending in the circumferential direction on which the balls are rolled. The concave surfaces of the first and second halves of the inner ring are so shaped that they form an annular groove with a U-shaped cross-section which opens radially outward when one of the halves comes close to the other half. The concave surfaces of the first and second halves of the outer ring are so shaped that they form an annular groove with a U-shaped cross-section which opens radially inward when one of the halves comes close to the other half.
The first half of the inner ring is secured to an input shaft. The second half of the inner ring is connected to the input shaft or the first half via a guide mechanism that converts rotational motion to reciprocating motion, such as a ball screw, and is urged toward the first half.
The halves of the outer ring are supported by a transmission casing and are rotationally fixed relative to the inner ring. That is, when the inner ring rotates together with the input shaft, the balls roll on the concave surfaces of the outer ring and rotate around the axis of the input shaft in the same direction as the rotating direction of the inner ring.
The first and second halves of the outer ring are connected, as in the case with the halves of the inner rings, via a guide mechanism that converts rotational motion to reciprocating motion, such as a ball screw, in such a manner that the distance between the two halves of the outer ring can be varied. A speed changing lever is attached to the first half of the outer ring. The speed changing lever protrudes from the first half of the outer ring. When the first half is rotated with respect to the second half by the speed changing lever, the distance between the halves of the outer ring is increased or decreased. For example, when the distance between the halves of the outer ring is decreased by operating the speed changing lever, the transmitting balls are pressed toward the inner ring by the halves of the outer ring and the second half of the inner ring is moved away from the first half against the elastic force of the spring. As a result, the radius of rotation of the balls around the axis of the input shaft is decreased.
A plurality of transmitting balls are interposed between the inner and outer rings and are in contact with a plurality of driven rollers interposed between the balls. The driven rollers are supported on an axial end of a roller supporting member. The roller supporting member is rotatable about the axis of the input shaft. That is, when the balls rotate around the axis of the input shaft, the driven rollers are pressed by the balls and the roller supporting member is rotated about the axis of the input shaft. An output shaft is connected to an end of the roller supporting member.
When the driven rollers rotate around the axis of the input shaft together with the transmitting balls, the output shaft is rotates with the driven rollers. Even if the rotational speed of the input shaft is constant, the rotational speed of the output shaft can be decreased by decreasing the radius of rotation of the transmitting balls and can be increased by increasing the radius of rotation of the transmitting balls around the axis of the input shaft. Thus, the rotational speed of the output shaft can be increased or decreased by operating the speed changing lever even if the rotational speed of the input shaft is constant.